I love manual focus lenses - even on AF cameras (I think there are not many 24x36mm DSLR cameras without AF, the Leica R9 with digital back, any more?).
One of my 5D is without AF, I have disabled it premanently, in favour to get a brighter viewfinder. AF taken away 2/3 of an f-stop.
I like the EE-S screens, my tests with a spilt image screen and fast lenses are bad. But screens need to be adjusted to be used with the very fast lenses.
I am very happy with my home modified Minolta Rokkor 58mm f/1.2. The Canon FD 50mm f/1.2L would be better - in technical terms - but I think I would not be as happy as with this legendary 58mm Rokkor.
The Contax Zeiss lenses are great, but some needs modifications on the lens and / or the camera mirror. For example my Zeiss 18mm/4 has to much protrusion. On the 25mm f/2.8 I need a very exact adapter to maintain infinity, and have no mirror crush.
Canon FD lenses are good performers - but need a kind of lens mount conversion. I have done several of these (20,24,85,100,300,400,500,800), and publicated some first instructions how to perform these conversions reversible, and how to perform the iris linkeage to the iris ring. FD modifications without prebuild parts is pretty much work.
There are many interessting M42 lenses out there, I love my Helios 44 58mm/2.0 lenses - they are cheap enough for some experiments
Most M42 lenses work well with an easy adapter on EOS 5D, some few protrude to much.
My most loved lens is the German Meyer Goerlitz Trioplan 100, with its very special bokeh
rendering:
https://photography-on-the.net …/showthread.php?t=1282872
Some Nikon F-mount lenses are interessting as well, and easy usable with adapters.
There are many hunderts of lens types around you can use with you EOS 5D! Some very easy, some needs much effort, some cheap for few USD, some cost a fortune.
I think I have ~200 lenses that work (proably with modification) on the EOS 5D - and there are still many many others around 